A robot, in modern parlance, comprises a general purpose manipulator, the movements of which are controlled by a 10 computer. In many cases, robots are designed to perform tasks in the past performed by humans. Thus, robots are required to perform gross movements, similar to those performed by the human arm, as well as precise movements similar to those performed by the human hand and fingers. In this regard, robotic systems typically employ a general purpose manipulator which includes two or more subsystems. The first of these subsystems may be termed a multi-axis manipulator. The second of these subsystems may be termed a head assembly which may include an end effector or centering head. It is an improved end effector or centering head to which the present invention is directed.
In a typical robotic system, the aforementioned multiaxis manipulator is utilized to provide gross movement control so as to generally position work or tool with respect to the workpiece. The head assembly is utilized to achieve precise movement control and to accurately position the work with respect to the workpiece.
While varying types of head assemblies are known, the type of head assembly to which the present invention is an improvement is required to provide linear movement, rotational movement and to provide simultaneous linear and rotational movement as well.
Such robotic systems have been found particularly suitable for the assembly of electronic equipment. It may be explained here that most electronic equipment includes electronic circuits formed by mounting various electrical components, such as resistors, capacitors, transistors, and integrated circuits, on a printed circuit board. The electrical components are mounted on the board with their terminals extending through holes in the board and connected to conductive paths on the surface of the board which connect the components in a desired circuit. Initially, such printed circuit board circuits were assembled by manually inserting the terminals of the components into the holes in the board. In order to increase the speed of assembling such printed circuit board circuits, robotic systems of the type here contemplated were developed which automatically inserted the components in the board.
One problem associated with prior art robotic systems which are used for electronic assembly purposes is that it is necessary that each component to be bonded to a given substrate be precisely aligned and positioned before bonding occurs. Different techniques have been employed to insure that the components to be assembled are properly aligned or oriented before bonding.
In one such technique, the components to be bonded are precisely aligned before being presented to the robotic system for assembly. For example, the components are sometimes precisely arranged on a carrier and the robotic system simply retrieves them from the carrier and thereafter bonds them to a substrate without changing the alignment or orientation of the components. This technique is relatively expensive, not only because it requires the presence of a carrier, but also because it requires the precise location of the various components with respect to that carrier.
In a second technique components are presented to the robotic system in a misaligned orientation rather than being aligned on a carrier. The robot selects a given component for assembly and thereafter transports that component to an alignment station at which the component is oriented. Thereafter the component is transferred to the substrate for assembly without change in orientation. This technique is unduly slow because it requires components to be directed to an additional work station, i.e., an alignment station rather than being directly applied to the substrate.
With a third technique, the disadvantages of the first two mentioned techniques are avoided. With the third technique components are presented to the robotic system with a misaligned orientation. The head assembly selects a given component and picks up that component with a pick-up means. With this technique, however, the head assembly is provided with an end effector, i.e., centering or alignment device, having fingers which not only grip but which also precisely align the selected component while the component is held by the pick-up means. The precisely aligned component is then bonded to the substrate without further change in its orientation. The head assembly of the robotic system which employs this third technique generally includes a hollow pick-up means or shaft through which a suction is drawn to engage and hold a component to the end of the pick-up means. Around the pick-up means are two pairs of fingers which are spaced ninety degrees apart, i.e., one pair of opposed fingers are utilized to align a component along the X-axis and the other pair are utilized to align the component along the Y-axis. The fingers are on the ends of arms which are pivoted adjacent their other ends so that the fingers can be swung along an arc toward and away from each other. When a component is picked up by the pick-up means, the fingers are swung together to center or align and engage the periphery of the component so that the component is grasped and held between the fingers.
A problem with this type of end effector or centering apparatus is that since the fingers move along an arc, they will engage different size components at different angles. Therefore, the fingers contact the component along what can be termed a line contact producing a very small contact area which results in deleterious stresses or pressures on the component. Also, such line contact can result in tilting of the components, particularly if the component is not of uniform size or shape, and poor positioning of the component with regard to the vacuum shaft. This can result in improper insertion of the component in the board.
A further problem is that the so-called X-axis fingers and Y-axis fingers are attempting to align or center the component simultaneously, e.g., the opposed pair of X-axis fingers engage the component to be centered and attempt to center the component while, at the same time, the opposed Y-axis fingers are in engagement with the component and are attempting to align the component. This results in a "tug-of-war" imposing unnecessarily high and undesirable forces on the component.
Also, the fingers are generally set or adjusted to handle the whole range of components which are expected to be encountered and the fingers are tailored to thinned down on at least one pair of opposed fingers to accommodate the whole range of components. On this latter point, for example, the opposed pair of X-axis fingers cannot engage the component if the Y-axis fingers are of the same size, i.e., have the same component engaging frontal area, as the X-axis fingers and the component is rectangular in shape. These two factors, i.e., setting the fingers to accommodate the range of expected components and the thinning down of the frontal engaging surfaces of one pair of opposed fingers, also result in undesirable forces being imposed on the components. Such forces, while only a few ounces, translate to thousands of pounds per square inch when small surface areas are involved as in centering chucks or end effectors of the type discussed above. These higher force may result in pressures on the components that cause internal defects, i.e., not visible, and cracks which can be seen. Such internal defects and cracks limit assembly reliability and result in low yields and generally, appear as electrical defects in the assembled electronic equipment.
Thus, there is a need to provide an end effector or centering chuck for use in a robotic system which does not suffer from these disadvantages and it is an object of this invention to provide such an end effector or centering chuck. It is a further object of this invention to provided an end effector or centering chuck which is capable of gripping, centering and locating components of varying size and shape with respect to a substrate without imposing unduly large forces on such components.
It is a still further object of the present invention to provide an end effector capable of the foregoing precise movements and which also grips or engages components with low impact force.
It is still a further object of the present invention to provide an end effector which has the benefit of low inertia due to its compact size and weight.